1. Field of the Invention
This invention relates to a differential device for producing difference between rotating numbers of two output elements, and to a drive power transmission unit using the same.
2. Description of the Related Art
As a conventional differential device for producing difference between rotating numbers of the two output elements, a device employing a planetary gear mechanism has been known in public as described in, for example, Japanese Patent No. 3247484. In the differential device disclosed in the Patent, torque of a electric motor is transmitted to two output elements through a planetary gear mechanism, whereby the difference generated between the two output elements can be limited or amplified.
The differential device disclosed in the aforementioned Patent, however, is provided with a gear of internal teeth type, so that it is required to machine the internal teeth and that the machining of the internal teeth is difficult in obtaining its accuracy. In the differential device disclosed in the Patent, the rotation of the motor is reduced by a gear connected between the motor and the planetary gear mechanism. Thus, a reduction ratio may be set at ⅙ approximately by that reduction mechanism, so that it may be difficult to obtain the larger reduction ratio thereby. In order to obtain sufficient torque for producing differential rotation, it may be required to improve output power of the motor or to increase the number of reduction gear element in a reducer. This improvement of the motor will cause enlargement of whole structure thereof, and the increase of gear elements in the reducer will also increase the number of parts thereof. Still further, the differential device disclosed in the Publication employs an internal gear in the planetary gear mechanism. Thus, bearings for supporting the internal gear need to be provided, whereby they cause a problem that the structure of the device is complicated.